linux: bump default to version 4.13.15

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

linux/Config.in

@@ -116,7 +116,7 @@ endif
 
 config BR2_LINUX_KERNEL_VERSION
 	string
-	default "4.13.13" if BR2_LINUX_KERNEL_LATEST_VERSION
+	default "4.13.15" if BR2_LINUX_KERNEL_LATEST_VERSION
 	default "v4.4.83-cip8" if BR2_LINUX_KERNEL_LATEST_CIP_VERSION
 	default BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE \
 		if BR2_LINUX_KERNEL_CUSTOM_VERSION